Galaxy formation and evolution

Abstract:

The growing number of stellar-mass binary black hole mergers discovered by Advanced LIGO and Advanced Virgo are starting to constrain the binaries' origin and environment. However, we still lack sufficiently accurate modeling of binary formation channels to obtain strong constraints, or to identify sub-populations. One promising formation mechanism that could result in different black hole properties is binaries merging within the accretion disks of Active Galactic Nuclei (AGN). Here we show that the black holes' orbital alignment with the AGN disks preferentially selects heavier black holes. We carry out Monte Carlo simulations of orbital alignment with AGN disks, and find that AGNs harden the initial black hole mass function. Assuming an initial power law mass distribution $M_{\rm bh}^{-\beta}$, we find that the power law index changes by $\Delta \beta\sim1.3$, resulting in a more top-heavy population of merging black holes. This change is independent of the mass of, and accretion rate onto, the supermassive black hole in the center of the AGN. Our simulations predict an AGN-assisted merger rate of $\sim4$Gpc$^{-3}$yr$^{-1}$. With its hardened mass spectra, the AGN channel could be responsible for $10-50$% of gravitational-wave detections.

Abstract:

We present an analysis of the gas outflow energetics from KMOS observations of ∼ 529 main-sequence star-forming galaxies at z ∼ 1 using broad, underlying H α and forbidden lines of [N II] and [S II]. Based on the stacked spectra for a sample with median star-formation rates and stellar masses of SFR = 7 M⊙   yr−1 and M⋆ = (1.0 ± 0.1) × 1010 M⊙, respectively, we derive a typical mass outflow rate of M˙wind = 1–4 M⊙ yr−1 and a mass loading of M˙wind / SFR = 0.2–0.4. By comparing the kinetic energy in the wind with the energy released by supernovae, we estimate a coupling efficiency between the star formation and wind energetics of ϵ ∼  0.03. The mass loading of the wind does not show a strong trend with star-formation rate over the range ∼ 2–20 M⊙ yr−1, although we identify a trend with stellar mass such that dM / dt / SFR ∝ M0.26±0.07⋆⁠. Finally, the line width of the broad H α increases with disc circular velocity with a sub-linear scaling relation FWHMbroad ∝ v0.21 ± 0.05. As a result of this behaviour, in the lowest mass galaxies (M⋆ ≲ 1010 M⊙), a significant fraction of the outflowing gas should have sufficient velocity to escape the gravitational potential of the halo whilst in the highest mass galaxies (M⋆ ≳ 1010 M⊙) most of the gas will be retained, flowing back on to the galaxy disc at later times.